Links in index of Qt help files not working anymore since Qt 5.15.0

The links in the index of Qt help files are not working anymore since Qt 5.15.0.
Using previous versions of Qt like 5.14.1 or 5.14.2, the index works properly.

This behavior was observed in the help files of the following applications, which use
Qt help files: Cppcheck, GNU Octave

Test builds of cppcheck, which show the behavior depending on the Qt version it was built against are provided here:
https://github.com/c72578/cppcheck/issues/1

The source code, how the Qt help file is implemented in Cppcheck can be found here:
https://github.com/danmar/cppcheck/blob/main/gui/helpdialog.cpp

How to reproduce under Windows:

• Start cppcheckgui.exe
• Press F1 to open the help file
• Click on Index
• Double click on the index entries, e.g. on "Error" or "Project"

Results

• Index is working, when cppcheck is built against Qt 5.14.1 or 5.14.2. The linked pages are shown.
• Not working, when built against Qt 5.15.0 or 5.15.1

Remark:

• The Contents of the Qt help files is working properly with all tested Qt versions (5.14.1, 5.14.2, 5.15.0, 5.15.1)

Under Linux, a broken index in the help file was also found in GNU Octave, when it is built against Qt 5.15.1
This is the case for the octave package in Fedora 33
rpm -qa octave
octave-5.2.0-7.fc33.x86_64
rpm -qa qt5-qtbase
qt5-qtbase-5.15.1-5.fc33.x86_64

How to reproduce:

• Install the octave package
  sudo dnf install octave
• run from terminal:
  octave --gui
• Click on
  Help - Documentation - On Disk
• Double click on any keyword in the "Function Index": No effect

Under Fedora 32, where we have Qt 5.14.2, the Function Index of the GNU Octave help file works properly.
rpm -qa octave qt5-qtbase
octave-5.2.0-1.fc32.x86_64
qt5-qtbase-5.14.2-5.fc32.x86_64

Remark: The Function Index in the Qt help file of GNU Octave contains lots of keywords and is quite important for looking up mathematical functions
The Qt help files are installed here under Fedora:
/usr/share/octave/5.2.0/doc/octave_interpreter.qch
/usr/share/octave/5.2.0/doc/octave_interpreter.qhc